The present invention relates generally to decreasing artifacts when a display is viewed off-axis.
The local transmittance of a liquid crystal display (LCD) panel or a liquid crystal on silicon (LCOS) display can be varied to modulate the intensity of light passing from a backlit source through an area of the panel to produce a pixel that can be displayed at a variable intensity. Whether light from the source passes through the panel to an observer or is blocked, is determined by the orientations of molecules of liquid crystals in a light valve.
Since liquid crystals do not emit light, a visible display requires an external light source. Small and inexpensive LCD panels often rely on light that is reflected back toward the viewer after passing through the panel. Since the panel is not completely transparent, a substantial part of the light is absorbed during its transits of the panel and images displayed on this type of panel may be difficult to see except under the best lighting conditions. On the other hand, LCD panels used for computer displays and video screens are typically backlit with fluorescent tubes or arrays of light-emitting diodes (LEDs) that are built into the sides or back of the panel. To provide a display with a more uniform light level, light from these point or line sources is typically dispersed in a diffuser panel before impinging on the light valve that controls transmission to a viewer.
The transmittance of the light valve is controlled by a layer of liquid crystals interposed between a pair of polarizers. Light from the source impinging on the first polarizer comprises electromagnetic waves vibrating in a plurality of planes. Only that portion of the light vibrating in the plane of the optical axis of a polarizer can pass through the polarizer. In a LCD the optical axes of the first and second polarizers are typically arranged at an angle so that light passing through the first polarizer would normally be blocked from passing through the second polarizer in the series. However, a layer of translucent liquid crystals occupies a cell gap separating the two polarizers. The physical orientation of the molecules of liquid crystal can be controlled and the plane of vibration of light transiting the columns of molecules spanning the layer can be rotated to either align or not align with the optical axes of the polarizers.
The surfaces of the first and second polarizers forming the walls of the cell gap are grooved so that the molecules of liquid crystal immediately adjacent to the cell gap walls will align with the grooves and, thereby, be aligned with the optical axis of the respective polarizer. Molecular forces cause adjacent liquid crystal molecules to attempt to align with their neighbors with the result that the orientation of the molecules in the column spanning the cell gap twist over the length of the column. Likewise, the plane of vibration of light transiting the column of molecules will be “twisted” from the optical axis of the first polarizer to that of the second polarizer. With the liquid crystals in this orientation, light from the source can pass through the series polarizers of the translucent panel assembly to produce a lighted area of the display surface when viewed from the front of the panel.
To darken a pixel and create an image, a voltage, typically controlled by a thin film transistor, is applied to an electrode in an array of electrodes deposited on one wall of the cell gap. The liquid crystal molecules adjacent to the electrode are attracted by the field created by the voltage and rotate to align with the field. As the molecules of liquid crystal are rotated by the electric field, the column of crystals is “untwisted,” and the optical axes of the crystals adjacent the cell wall are rotated out of alignment with the optical axis of the corresponding polarizer progressively reducing the local transmittance of the light valve and the intensity of the corresponding display pixel. Color LCD displays are created by varying the intensity of transmitted light for each of a plurality of primary color elements (typically, red, green, and blue) that make up a display pixel.
LCDs can produce bright, high resolution, color images and are thinner, lighter, and draw less power than cathode ray tubes (CRTs). As a result, LCD usage is pervasive for the displays of portable computers, digital clocks and watches, appliances, audio and video equipment, and other electronic devices. On the other hand, the use of LCDs in certain “high end markets,” such as medical imaging and graphic arts, may demand an even greater dynamic range than available with cathode tube backlight based LCDs.
Another type of LCD display construction is to include a light emitting diode based backlight array. Such an array permits the individual selection of the luminance for individual elements of the backlight array. By selective illumination of the individual elements, different regions of the display may be selectively dimmed or otherwise turned off, which increases the dynamic range of the display.
Whatever configuration is used for the liquid crystal display, they generally have somewhat reduced performance when viewed from oblique directions. This reduced performance may manifest itself, for example, as decreased contrast, incorrect color rendering, and increased image artifacts. In many cases, some of these performance reductions are more pronounced at lower luminance levels. Residual light leakage, especially in oblique directions, also tends to limit the contrast range of the display at lower light levels.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.